The present invention relates to a driving method of a liquid crystal display device using a liquid crystal material having spontaneous polarization and also relates to a liquid crystal display device adopting the driving method.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. Further, with the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices have been widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
The liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) provided on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility resulting from the reflected light amount that varies depending on environmental conditions, the transmission type liquid crystal display devices are generally used as display devices of, particularly, personal computers displaying a multi-color or full-color image.
In addition, the current color liquid crystal display devices are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type, based on the liquid crystal materials to be used. The STN type liquid crystal display devices have comparatively low production costs, but they are not suitable for the display of a moving image because they are susceptible to crosstalk and comparatively slow in the response rate. In contrast, the TFT-TN type liquid crystal display devices have better display quality than the STN type, but they require a back-light with high intensity because the light transmittance of the liquid crystal panel is only 4% or so at present. For this reason, in the TFT-TN type liquid crystal display devices, a lot of power is consumed by the back-light, and there would be a problem when used with a portable battery power source. Moreover, the TFT-TN type liquid crystal display devices have other problems including a low response rate, particularly, in displaying half tones, a narrow viewing angle, and a difficult color balance adjustment.
Therefore, in order to solve the above problems, the present inventors et al. are carrying out the development of a liquid crystal display device using a ferroelectric liquid crystal having spontaneous polarization and a high response rate of several hundreds to several μs order with respect to an applied voltage. When a liquid crystal material having spontaneous polarization is used as the liquid crystal material, the liquid crystal molecules are always parallel to the substrate irrespective of the presence or absence of applied voltage, and the change in the refraction factor in the viewing direction is much smaller compared with the conventional STN type and TN type. It is thus possible to obtain a wide viewing angle. Moreover, in a liquid crystal display device in which a ferroelectric liquid crystal that is superior in the response characteristics and the viewing angle to the conventional liquid crystal materials is driven by a switching element such as a TFT, it is possible to achieve a light transmittance corresponding to the magnitude of the applied voltage and display a half-tone image and a moving image.
This ferroelectric liquid crystal has the applied voltage-light transmittance characteristics as shown in FIG. 1. More specifically, the light transmittance of the ferroelectric liquid crystal varies depending on the polarity, and, for example, when a positive voltage is applied, the light transmittance is increased according to the applied voltage, while when a negative voltage is applied, the light transmittance becomes zero irrespective of the magnitude of the applied voltage. Accordingly, in the conventional example, display is controlled by a drive sequence as shown in FIG. 2.
In one frame for forming a display image, selective scanning is performed twice for the pixel electrodes of each line, and voltages of equal magnitude and opposite polarities are alternately applied to the liquid crystal material at a predetermined cycle and for a predetermined period. The magnitude of the applied voltage corresponds to the image data, and a display image is obtained (writing is performed) by applying a voltage corresponding to the image data at the beginning of each frame, and then the display image is erased (erasure is performed) by applying a voltage having different polarity and the same magnitude as the above voltage. By repeating such writing and erasure in each frame, the display of image is realized.
In this driving method, as shown in FIG. 1, when the applied voltage has the negative polarity, the transmittance is substantially 0%, and thus black display is implemented. Therefore, the time contributing to actual display is a half of the total time, and there is a problem that the light utilization efficiency given by the ratio of the screen brightness to the light source brightness is low (the screen brightness/back-light brightness percentage is 6% in the conventional example adopting the drive sequence shown in FIG. 2).
Furthermore, since the ferroelectric liquid crystal has spontaneous polarization, it is necessary to store charges twice more than the spontaneous polarization in each pixel electrode for selective scanning of each pixel electrode, and thus there is a problem that a liquid crystal material having large spontaneous polarization can not be used in view of the facts that the capacity of each pixel electrode and the drive voltage are not so high.
Besides, when the incorporation of the liquid crystal display device into a portable apparatus is taken into consideration, it is preferred to drive the liquid crystal display device by a low voltage, but there is a problem that driving by a sufficiently low voltage has not yet been realized (the drive voltage is 12 V in the conventional example using a ferroelectric liquid crystal having spontaneous polarization of 11 nC/cm2).